The range of Sauros HAs is enoough to keep them away from the walls while shooting at your units. And normaly, their troops immidietly move away and take positions far away from the gates, but unlike other infantry based armies they are permanently on skirmishing mod if you attack them with anything else then skirmishers/archers. And while you are moving forward they´ll shoot your poor armoured troops down, since shield value 3 of most of your units won´t even protect them from the front, so they will be shot to pieces...